What I want to do
in this video is talk a little bit about maybe
why the plates are actually moving in the first place. And nothing I'm talking
about in this video has been definitively proved. This is just kind of the current
thinking, the leading thinking, on why plates are
actually moving. Although we haven't seen
the definitive evidence yet, and it's probably a combination
of a bunch of things. Now, before we even
talk about plates, let's just talk
about convection. And you might already be
familiar with the term, but just in case
you're not, let's do a little bit of
review of convection. So let's say I have
a pot over here. So that is my pot. And it contains some water. So I have water in my pot. And let's say I only
heat one end of the pot. So I put a flame right over
at that end of the pot. So what's going to happen? Well, the water that's
right over the flame is going to be warmed up more
than any of the other water. So this water is
going to get warm. But when it gets warm, it
also becomes less dense. When you have a fluid,
if you warm it up, the molecules are
vibrating more. They have more kinetic energy. They're going to a bounce
further distances away from each other. They will become less dense. And if you have something
that's less dense, and it's surrounded by
things that are more dense, and we're dealing in kind
of a fluid state right here, that warm, less dense water
is going to move upwards. It's going to move upwards. Well, when it moves upwards
something has to replace it. So you're going to
have cooler water from this side of the
container kind of replacing where that water was. Now, this water, as it rises,
what's going to happen to it? Well, it's going to cool down. It's going to get
further from the flame. It's going to mix with maybe
some of the other water, or transfer some of
its kinetic energy to the neighboring water. So it'll cool down. But once it cools down,
what's it going to want to do? Remember, in general, the
closer you are to the flame. So the water closer to
the flame in general is going to be warmer. So all of this stuff
is going to be warmer, and all of this stuff up
here, like the coldest water, is always going to be
furthest from the flame. And so the coldest water
is going to be over here. But remember, the coldest water
is also the densest water. So this water over
here is dense. And so it will sink. It's denser than
the water around it. And it also helps
replace the water that's going here to
get warmed up again. And so what you do is
you have this cycle here. Warm water rises, moves
over to the right down here, and then goes back down as it
cools down, and it's dense, and then it gets
warmed up again. And so this process,
essentially what it's doing is it's transferring the heat. It's allowing the heat to be
transferred from this one spot throughout the fluid. And so we call this
process, this is convection. Now, the reason why we
think the plates are moving is because we think that there
are similar types of convection currents in the
asthenosphere, in the mantle, in the more fluid
part of the mantle. Remember, most of the mantle
you can kind of view it as this mooshy, spongy, not
quite liquid, but not quite solid state, kind of plastic. It can kind of moosh past. It can kind of flow like super,
super, super thick, like super viscous fluid. So not quite solid,
not quite liquid. But the same thing
could be happening. You have certain areas
in the mantle that are hotter than others,
and this particular in the asthenosphere. And those areas,
that's where you're going to have the material
in the mantle move up, because it's
hotter, it's less dense, and it will move up. And maybe it'll cause one
of these divergent rifts where kind of plate material
and crustal material is forming. And then as it moves
up, it cools down, and eventually sinks, only
to get heated up again. So it has this kind
of circular motion just like we saw with
the boiling water. And so that process, remember
this isn't completely liquid. It is rocky. So it's going to
potentially be able to take other things with it. It could maybe drag the
crust along with it. Or I should say it could
drag the lithosphere along with it, not just the crust. It could drag all this rigid
rock up here along with it, causing it to move in
that general direction. So you have the drag there. You could also imagine this
kind of a suction effect, where, if you view
it as a fluid, you have a bunch of
fluid coming down here. So it would kind of pull
the lithosphere down at those points, and it would
kind of push the lithosphere up at those points. So you have these
convection currents that are essentially driving it. These aren't going to be super
fast moving fluid convection currents, like you would
expect with boiling water or with heated water. These would be slow moving
convection currents, but they're moving
enough, and they're able to kind of put enough
drag on the lithosphere to take the lithosphere
along with it. And so that's kind
of at a high level the dominant theory as
to why they're moving. There's other ones
that talk about maybe these lithospheric plates,
they kind of thicken as they move further
away from this area where the ridge is forming. And if we look at this
oceanic crust right over here. And so over time,
they're denser over here, because there's more cooled
down material at these points. And it's already
a little bit lower because this is where a lot
of the land is being created. And so there's maybe some
gravitational effects. But the dominant
effect, we think, is due to this convection
in the upper mantle.